1. Field of the invention
This invention relates to a process for forming a deposited film containing germanium, above all a functional film, particularly an amorphous or crystalline deposited film containing germanium to be used for semiconductor device, photosensitive device for electrophotography, line sensor for image input, image pick-up device, photovoltaic device, etc.
2. Description of the Prior Art
For example, for formation of an amorphous silicon film, the vacuum deposition method, the plasma CVD method, the CVD method, the reactive sputtering method, the ion plating method, the optical CVD method have been attempted to be practiced, and, in general, the plasma CVD method has widely been used and industrialized.
However, for the deposited film constituted of amorphous silicon, there is room left for further improvement of overall characteristics with respect to electrical, optical characteristics and, fatigue characteristic or use environmental characteristic in repeated uses, further productivity and mass productivity inclusing uniformity and reproducibility.
The reaction process in formation of amorphous silicon deposited film according to the plasma CVD method generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and not a few ambiguities existed in its reaction mechanism. Also, there are involved a large number of parameters for formation of its deposited film (e.g. substrate temperture, flow rates and their ratios of gases introduced, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, gas discharging speed, plasma generation system, etc.), and the plasma formed by combination of such a large number of parameters may sometimes become unstable to frequently give markedly bad influences to the deposited film formed. Besides, the parameters inherent in the device must be chosen for each device, and it has been difficult under the present situation to generalize the production conditions. On the other hand, for exhibiting electrical, optical or mechanical characteristics of an amorphous silicon film satisfactorily for respective uses, it has been deemed best to form an amorphous silicon film according to the plasma CVD method under the present situation.
However, depending on the applied uses of the deposited film, since it is required to satisfy sufficiently enlargement of area, uniformization of film thickness, uniformity of film quality, and also to attempt to perform a mass production with reproducibility by a high speed film formation, enormous equipment capital becomes necessary for mass production devices in formation of amorphous silicon deposited films according to the plasma CVD method, and the management items for mass prodution thereof will become complicated to make the management tolerance narrower. These matters, and also subtlety in adjustment of the devices, have been pointed out as the problems to be improved in the future. On the other hand, in the conventional CVD methods of the prior art, high temperture is required to be used and no deposited film having practical characteristics could be obtained.
As described above, in formation of amorphous silicon films, it has earnestly been desired to develop a formation process which can perform mass production by means of a low cost device while maintaining practical characteristics and uniformity. These discussions may also be applicable to other functional films such as silicon nitride films, silicon carbide films, silicon oxide films, etc.